Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
Among the available wireline and LWD tools are a variety of resistivity logging tools including, in particular, “laterolog” tools. Such tools typically include a central electrode around a tool body, with guard electrodes symmetrically spaced above and below the central electrode. The tool drives auxiliary currents between the guard electrodes and the center electrode to “focus” the current from the center electrode, i.e., to reduce dispersion of the current from the center electrode until after the current has penetrated some distance into the formation. Generally speaking, a greater depth of investigation can be achieved using more widely-spaced guard electrodes, but the vertical resolution of the measurements may suffer.
Laterolog tools employ a monitor electrode array to sense voltages resulting from injected currents. These voltages are analyzed to determine formation properties (e.g., resistivity) at different depths. Such measurements are affected by formation properties, borehole properties, and controllable parameters such as the excitation current level, the excitation current frequency, and amplifier gains. Laterolog tools that do not adequately manage controllable parameters may cause inaccuracies in the determination of formation properties.
It should be understood that the drawings and detailed description do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed in the scope of the claims.